1. The problem statement, all variables and given/known data A particle with momentum p0, mass m0 and energy E0 decays into two particles with mass m1 and m2. Find the energy of the particle E1 and E2. 2. Relevant equations Four-momentum! 3. The attempt at a solution I calculated the energy of particle 1 in S' (system where particle 0 is at rest) in dependence of m0', m1', m2'. I got also the momentum of the particle 1 in system S' which I can write in dependence of m0', m1', m2' and E1'. For example I have taken the coordinates of system S' in such a way, that I can write the momentum p'1 of particle 1 as: p'1=(0,py',0) with py'=sqrt(f(m0',...)). Thus getting the four-momentum of particle 1: (E', 0, c*py', 0) Well, what my question concerns. How can I translate the whole thing back into system S? Lorentz-Transformation or is there a easyer way? How can I go on to fullfill the task?